1. Field of the Invention
The present invention relates generally to the fields of synthesis of polyacrylonitrile polymers comprising ionic comonomers and the substitution of active derivatizing agents onto the available sulfate, sulfonate, carboxylate, or their respective acid functional groups. The derivatizing agents can be protonated amines, which displace the sodium, hydronium, or other cation bound to the functional groups, and become strongly bound to the polymer via an ionic bond. The method is useful because, among other reasons, 1) many protonated amines are antimicrobial agents that will impart antimicrobial activity to the polymer, 2) these derivatizing agents remove sodium, which is especially important if the fibers are to be carbonized, 3) the derivatizing agents can strongly affect whether the polymer is hydrophilic or hydrophobic, and 4) these derivatives can improve the rheological properties of the polymer and make microdenier fiber production possible.
2. Description of Related Art
Acrylonitrile and its comonomers are polymerized by any of several well-known free-radical methods. All commercial processes are based on free radical polymerization because it gives the combination of polymerization rate, ease of control, and properties including whiteness, molecular weight, linearity, and the ability to incorporate desired comonomers and, in most cases, dye sites. The most widely used method of polymerization in the acrylic fibers industry is aqueous dispersion polymerization which is a variant of suspension polymerization. Radical fragments generated by redox catalysts in this type of process contain a sulfate or a sulfonate functional group. Polyacrylonitrile particles made by this process tend to grow primarily by agglomeration of smaller particles.
Nearly all acrylic fibers are made from acrylonitrile copolymers containing one or more additional monomers that modify the properties of the fiber. Neutral comonomers including methyl acrylate, methyl methacrylate, or vinyl acetate are used to modify the solubility of the acrylic copolymers in spinning solvents such as dimethyl acetamide, to modify the acrylic fiber morphology, and to improve the rate of diffusion of dyes into the acrylic fiber. Despite its disadvantages of low reactivity and difficulty in polymer control and chain transfer in polymerization, vinyl acetate is increasingly the comonomer of choice for acrylic fibers, primarily because of its low cost.
Dyes attach to the polymer at end groups and where ionic functional groups are available. Generally, fiber dyeability is critically dependent on the molecular weight distribution of the polymer because most acrylic fibers derive their dyeability from sulfonate and sulfate activator and initiator fragments, respectively, at the polymer chain ends. Thus, the dye site content of the fiber without the presence of an ionic comonomer is inversely related to the number average molecular weight of the polymer and very sensitive to the fraction of low molecular weight polymer. With the trend to finer denier fibers where more dye is required to achieve a given color, the need for dye sites is increased. Over the years, many producers have gradually lowered the molecular weight of their polymer to increase dyeability. The total number of dye sites required to be able to dye a full range of shades with cationic dyes is 30 to 50 milli-equivalents per kilogram (meq/kg) depending on the fiber denier and structure. Dry-spun fibers and microdenier fibers require a minimum of 40 meq/kg of dye sites.
Where the number provided by the end groups is inadequate, an ionic comonomer may be used to provide additional dye sites within the polymer structure. Ionic comonomers that contain sulfonate groups, such as sodium p-styrenesulfonate, sodium methallyl sulfonate, sodium p-sulfophenyl methallyl ether, or sodium 2-methyl-2-acrylamidopropane sulfonate, may be added to provide dye sites apart from end groups and to increase hydrophilicity. Carboxylate-containing comonomers such as itaconic acid and acrylic acid have also been employed as dye receptors. These dye site, or ionic, comonomers contain a carboxylic acid or sulfonate functional group and a polymerizeable vinyl hydrocarbon functional group.
It is known that protonated amines can impart antimicrobial activity to polyacrylonitrile polymer. Early technology involved applying a protonated amine or other antimicrobial agent topically. The topical treatment was subject to degradation by abrasion and by washing cycles. Another treatment involved binding an alkoxysilane quaternary ammonium salt from a methanol solution to fibers or to finished product. The bond was by hydrolysis and condensation of the alkoxysilane with hydroxyl groups in the fiber. But acrylic fibers do not contain appreciable hydroxyl groups to which the alkoxysilane groups can bind. There were also problems of interactions with emulsion-based finishes used in the textile industry and deactivation by soaps.
Pardini in U.S. Pat. No. 4,708,870 suggested that non-fugitive antimicrobial activity can be imparted into acrylic polymers by copolymerizing a comonomer that contains a protonated amine. Such monomers include, for instance, dimethylamino-ethylmethacrylate. Benefits of this treatment were that the antimicrobial agent was covalently bound to the polymer and was therefore not fugitive, and that the antimicrobial agent was present throughout the polymer. A problem with this technology is that the protonated amines are subject to attack from subsequent textile treatments. For instance, cationic antistatic agents (antistats) were required for this polymer as anionic antistats would react with the protonated amine. There was also the problem of wasted antimicrobial monomer, because many applications do not need antimicrobial agents within a fiber. Finally, the antimicrobial activity was fixed, both in the compound used and in the quantity incorporated, upon making the polymer. Different final uses require different degrees of antimicrobial activity and incorporating the antimicrobial agents when the polymer was being made did not provide the flexibility needed by industry.